In spread spectrum communication systems, the information to be transmitted is encoded with a pseudorandom or pseudo-noise (PN) code. A pseudo-noise code is a serial stream of logic ones and zeros generated by a clocked shift register having feedback connections from selected stages through exclusive-OR gates to the input of the register. The sequence of ones and zeros of the bit stream appears random over the short run but, in reality, is a repeating sequence. The length of the bit sequence before repetition occurs is determined by the number of stages in the shift register. The manner of combination of the PN code with the information signal is by modulo-2 addition which is identical to the operation performed by an exclusive-OR gate.
The modulation of the information signal by a PN code causes the modulated signal to occupy a much greater bandwidth than would be required by a carrier signal modulated by the information signal alone. Because of the wideband signal spectra generated by PN code modulation, the power transmitted is low in any narrow band of frequencies. In order to demodulate a spread spectrum coded signal, an exact replica of the PN code must be combined in proper synchronism with the coded signal. This requirement prevents interception of the information signal by a casual listener. For this reason, spread spectrum encoding is often employed, along with other measures, to encrypt confidential messages.
Heretofore, spread spectrum techniques have been employed principally in radio communications. The use of spread spectrum encoding on wired communication facilities has been limited because of the relatively narrow bandwidth of most wired transmission lines. Known uses of spread spectrum techniques on wired transmission lines include spread spectrum encoding of voice signals, that is vocal scrambling, to secure telephone conversations and the provision of spread spectrum encoded supervisory signals on dedicated telephone lines used for interconnecting local burglar alarm stations with central stations.